I am running a 1-shot which is going to be coming up to an encounter with a mage who has a custom set of spells (to theme him better) and the ability to cast level-1 vicious mockery (IE 1d4 damage) on everyone each turn by making terrible puns. It's a very serious one-shot.

I am concerned that the final showdown will be over far too quickly. Here's the breakdown:

The party at level 5:

- A Bard who's college has thus far eluded me. They've used invisibility, bardic inspiration and vicious mockery thus far.
- a werebear half-orc barbarian (bear totem).
- a half-orc druid, thus far has spent most of the game as a giant spider
- a bronze dragonborn fighter carrying most of an armoury on her person, tends to go for battleaxe and shield combo
- a human paladin/artificer who can do a lot of damage with Booming Blade & Smite

The BBEG:

 This can all be tweaked to make him more of a threat, as I'm now realising that there are some issues with his spell list - the 3 main ones he would be using (Black tentacles, Fly, and Animate Objects) are all concentration spells. I may have to give him winged boots to compensate, but we will see.

At any rate, my biggest concern is that he only has 60hp, and as such may not even get a single round of combat if he rolls poorly! I don't want to have him feel super buff, though - I want it to feel like he's magical, not like he's a tank. I am considering giving him a "shield" which would be a pool of temporary hitpoints right at the start to make the fight last but to also make him feel squishy without being squishy - I was seeing regular high-20's of damage each turn, so I don't doubt that just 2 of the players could take him out in one turn at the moment.

He has 2 medium-sized water weirds (27hp, AC12, resistant to nonmagical attacks) which will be in a position to intercept the party on their way up to the top of the tower where he is, which should buy him some time to let off some spells. 

Do you think that he will just fall down in one round? Or is he about right with 2 mini-weirds against 5 level 5's?

I don't know your party to answer your question about how he will fare after the mini-weirds. But I can offer some insight into how to make him seem bigger than he might be. 

• If you think it needs more hitpoints, you're probably right. You may need to triple their HP or quadruple it, just to get them to stand longer. The idea of temp HP pools isn't a bad one, maybe a temp HP pool refresh as a lair action on initiative count 10 (losing ties). (Default being that the party would have to fight them in their lair.) 
• Another method of HP regeneration might be the Mythic Monster traits, which (for the most part) allow the creature to trigger a HP refill with some type of physical attribute change and then do something like move up to their movement (no Opportunity Attack trigger) or some large one use ability like Synaptic Static.
• Mage armor should be applied and not count against the amount of spells they can cast. Just my 2cp here but, shield and counterspell might be served better as reaction abilities, instead of spell slots to consume. We only plan on this thing living for 3 rounds anyway, and  they only get one reaction per turn. May also want to consider misty step this way as well. Either as a straight BA, or a Legendary (1 Action).
• Allow a Legendary (1 Action) to cast a Cantrip or MM (1st level only).
• Your Cheesy one-liners (AKA Bad@$$ Dad Jokes) might need to trigger on initiative count 20 (losing ties) typical of a Lair Action so as to not eat up the NPC's action/reaction/bonus action. You could combine the HP refresh and this ability under the ability: Working the Room
• You will want something that it can call for "help". Really they aren't much help, other than getting underfoot and forcing the PCs to break focus on the BBEG. Maybe allow calling them to be a Recharge 6 (d6) action to get 1 or 2d4 "minions" to rush to its aid. You could call em bouncers or security goons. Give them one HP, static damage of about 7-10 and around a +5 or +7 to hit. If they are hit with a save for half damage effect, they save for none. Might give BBEG the last Legendary (2 Actions) to motivate those minions that have fallen to rise to their feet and fight on. 

Hope this gives you something to work with, maybe something will peak your inspiration. Have fun, and good luck!

Thanks for the reply!

I am going to replace the Animate Objects spell to instead make minions, so no concentration needed. I already have just such a minion for the task!

I have given him 40 temporary HP at the start of the fight, that should let him take a round of hits without going down.

Cheesy one-liners are a free action so he basically says them as he does stuff on his turn, for free.

I've also realised he's sort of one of a pair of BBEGs, as he has summoned a big beasty to destroy the town and that is going to also have a lot of HP to get through (but none of the spells) so I cn always have him make a retreat to return and attack when they fight said big beasty!

IMO, with the party makeup and damage potential, I don't see him lasting beyond round 2, even with the 40 temp HP.  For an enemy who's supposed to last a few rounds, at the level the party is at, I usually start around 200 and depending on how tough I want him, add or slightly reduce, counting as well, the damage he can dish out.  A combination of relatively low AC and low HP is the kicker.  Keep in mind, your BBEG isn't under any of the restrictions for how beefy he is compared to your party.  In order to extend the fight and give it some meaning and impact for the party, you may find you want to boost his HP.  In rare occasions, I have done this on the fly, seeing my boss HP plummet too quickly, I just add some more.....die rolls are the normal culprit, slashing away health much faster than I want, for a "big' fight.

Quote from Falwith >>

IMO, with the party makeup and damage potential, I don't see him lasting beyond round 2, even with the 40 temp HP.  For an enemy who's supposed to last a few rounds, at the level the party is at, I usually start around 200 and depending on how tough I want him, add or slightly reduce, counting as well, the damage he can dish out.  A combination of relatively low AC and low HP is the kicker.  Keep in mind, your BBEG isn't under any of the restrictions for how beefy he is compared to your party.  In order to extend the fight and give it some meaning and impact for the party, you may find you want to boost his HP.  In rare occasions, I have done this on the fly, seeing my boss HP plummet too quickly, I just add some more.....die rolls are the normal culprit, slashing away health much faster than I want, for a "big' fight.

 Yeah, I agree with mid combat adjusting. 

If you don't want to make your BBEG into a simple tank, your other option is to make it so it's hard for the PCs to attack -- barriers, control effects, high mobility, etc. Given the classes, you mention, Just having him cast Transmute Rock to turn the obvious attack path into a morass should buy him multiple rounds to harass the PCs.

To quote Megamind, "PRESENTATION!"

Give the BBEG, for lack of a better word, flair. Give them opportunities leading up to the fight to present their powers and skills. Allow them to communicate with the players and display their strength.

In my LMoP+DoIP campaign, I've had my players encounter the White Dragon Cryovain several times, nearly killing them. When they infiltrated the lair at 3rd level, they chose to not wake the dragon, for fear of it killing them.

If your players are scared of the BBEG before the final battle, it will make them feel more powerful within the battle.

Thanks for all the replies!

I think that I'll aim to give him enough temporary HP to let them break through on round 2, then gauge his HP to make him last 'til the end of round 3 or early round 4 - assuming he doesn't just delete the party, he does have fireball after all! I also want the players epic combos to feel like they do something, so I guess if he is out of temporary HP and down by 30hp or more then he can probably be finished off if someone lands a huge combo on him.

Presentation is alas a difficulty as he is in a tower doing nasty magic shenanigans, and the party are on their way up to the tower to find out what is happening and then fight him, so for a one-shot I can't do too much foreshadowing of the BBEG!

1. Run it as you have it in the OP
2. When he reaches 0hp -- CUTSCENE!
3. Make a whole new statblock for battle #2

First fight is a bit reserved and tactical, second one is desperate and brutal. Maybe he's made some dark pact that sustains his body at a terrible price, or maybe the first battle was actually his apprentice. There's a million ways to frame it, but splitting the fight up into stages is a great way to set it apart while also preventing the party from just steamrolling it in round 1.